1. Field of the Invention
This invention relates to recording of seismic data by individual recording units located remotely from a central station.
2. Description of the Prior Art
During the course of seismic exploration, a plurality of seismic transducers or transducer groups are emplaced at desired intervals along a line of survey. The respective transducers are coupled to corresponding input channels of a multichannel recording system in a central station. The input channels include filters and signal-conditioning amplifiers. Signals from the transducers are processed through the input amplifiers, digitized and multiplexed to a recording medium such as magnetic tape. The transducer signals may be transmitted over land lines through a multiconductor cable wherein each transducer and corresponding input channel are interconnected by a dedicated wire pair. In another arrangement, the seismic signals are transmitted via a single-channel telemetric system using time division multiplexing. The transducers are generally spaced 200-300 feet apart. Up to 100 transducers and input channels may be used. Thus, several miles of cable must be laid out if land lines are used.
A number of systems are known or have been proposed to eliminate the need for the many miles of interconnecting cable. These systems either transmit seismic signals from the respective transducers to the central station by radio or the seismic signals are processed and recorded locally at each individual transducer, operating under radio commands from the central station. In such systems, some of the data-processing electronics are removed from the central station and installed in remote modules, one of which is associated with each of the transducers or transducer groups. The remote module may include a recording device such as a cassette tape for recording seismic signals resulting from several seismic shots during a recording period.
Representative known radio-controlled remote seismic data recorders are disclosed in U.S. Pat. Nos.: 3,062,315 to Herzog; 3,075,607 to Aitken et al; 3,283,295 to Montgomery; 3,288,242 to Loeb; 3,806,864, 3,987,406, and 4,010,442 all to Broding; 3,886,494 to Kostelnichek; 3,946,357 to Weinstein; and 4,042,906 to Ezell.
In the above-listed systems, selected units are turned on after receipt from the central station of a coded radio command. The coding determines the selection of the remote modules to be activated. To synchronize the various recorders, one with the other, and to provide accurate timing during a recording cycle, timing pulses are also transmitted to the remote modules. In some of the systems above, the recorded seismic data may be played back via radio to a master recorder in the central station for permanent storage. In others of the above systems, at the end of a recording period such as at the end of a day's work, the cassette tapes are harvested from the various recording modules and are played back either at the central recorder or in a data processing center.
The problems with the known art employing remote radio-controlled units are manifold. Each individual recording module must be separately addressed by a suitable radio code. This requirement necessitates complex coding-decoding circuitry in both the central station and in each individual module. The individual identification numbers of the respective modules must be recorded in the header of each seismic recording. If the data recorded at each module are to be played back over a radio link, then each module must play its data back in sequence because there are not available enough separate radio-channel frequencies to play back 100 recordings in parallel. Other remote playback techniques such as time-division or frequency-division multiplexing over a limited number of channels add extra complexity to the system. Furthermore, in known systems it is essential to transmit separate timing signals for accurate time synchronization of the remote recording modules, one with the other and with the central station. Additionally, many radio-controlled systems use radio frequencies that are effective only along line-of-sight. Therefore they are often not effective in mountainous or obstructed terrain, the very environment in which remote individual recording units are most useful. And finally, the requirement for at least a radio receiver, if not also a data transmitter, plus an antenna at each remote recording unit adds substantial complexity and bulk to the units.